This invention concerns contact lenses. In particular, the invention concerns an improved system and design for aligning a contact lens relative to the eye.
Hydrogel or xe2x80x98softxe2x80x99 contact lenses are used to correct refractive errors of the human eye. One type of such lenses is the spherical power hydrogel lens, which is used to correct myopic or hyperopic refractive errors of the eye. In these lenses, the optical power is homogeneous. The rotational orientation of such lenses is non-specific and has no impact on the correction of refractive error. Visual performance of the lens does not suffer from changes in rotational orientation with respect to the eye. In other words, the orientation of the lens in the eye is not important, because the optic zone of the lens itself is radially symmetrical, and not designed to rest on the surface of the eye in a specific orientation.
A second type of more specialized hydrogel contact lenses is required for patients who seek to correct astigmatism refractive errors of the eye using hydrogel contact lenses. Such contact lenses are sometimes referred to a xe2x80x98toricxe2x80x99 contact lenses. Hydrogel lenses that correct astigmatism refractive errors contain a cylindrical power component and a spherical power component. Because the spherical and cylindrical powers are in meridians which are orthogonal to one another, the orientation relative to the eye of a hydrogel contact lens which seeks to correct astigmatism errors in the eye becomes important. The rotational orientation of an astigmatism hydrogel lens in the eye requires alignment along a specific axis. If the rotational orientation of an astigmatic lens is permitted to vary from this axis, visual performance of the lens may suffer, and the desired corrective effect of the lens may be lost. Numerous existing design systems have been used to stabilize astigmatic hydrogel lenses on the eye in an effort to correct the refractive error and maximize visual performance.
The current design systems used to stabilize hydrogel astigmatic contact lenses include prism ballast, periballast and xe2x80x9cthin-zonexe2x80x9d (shown in FIG. 1). These stabilizing design systems each have at least one characteristic in common. These lens designs have a disparity in thickness between the superior periphery and other regions of the contact lens. The disparity is designed to orient the lens with each blink, as the thinner portion of the lens moves naturally towards the top of the eye with the natural blinking action. Relative to the inferior eyelid, the superior eyelid makes the greatest excursion during the blink. Such a contact lens orients on the eye in a rotational configuration that minimizes resistance to the blink. This orientation is with the xe2x80x9cthin zonexe2x80x9d in a superior position. The disparity in lens mass between the superior xe2x80x9cthin zonexe2x80x9d and the thick regions of the lens is directly related to rotational stability on the eye. The greater the difference in lens geometry the more rotationally stable the lens is on the eye. This disparity in thickness is not desirable, however. Such lenses may also be less comfortable due to the disparity in thickness.
In addition, the thicker portions of current design systems for stabilizing contact lenses may interfere with the natural metabolism of the eye. The cornea is the ocular surface upon which hydrogel lenses are worn. The tissue of the cornea requires oxygen to maintain normal metabolism. The cornea is avascular and therefore has no direct blood supply for its oxygen requirement. The major source for corneal oxygen is from the atmosphere. The materials from which hydrogel lenses are fabricated are permeable to gases, and in particular, oxygen. The ability of a particular hydrogel lens to pass oxygen to the cornea from the atmosphere is called oxygen transmissibility. This property is a function of material and thickness. Therefore, hydrogel thickness and oxygen transmissibility are inversely related. Oxygen transmissibility and lens rotational orientation are inversely related with current design systems. Thus, the additional thickness in a hydrogel contact lens used to orient the lens inhibits the transmission of oxygen to the cornea, and interferes with the metabolism of the eye.
The present invention is concerned with providing a novel system for stabilizing the rotational orientation of a contact lens.
Another object of the present invention is to decrease the necessary thickness of material used in contact lenses which require a specific rotational orientation.
Yet another object of the present invention is to increase the comfort level for users of contact lenses which require specific rotational orientation.
Still another object of the invention is to permit the manufacture of contact lenses requiring a specific rotational orientation using a smaller amount of material (such as hydrogel) used in the manufacture of the lens, an advantage which lends itself to mass production of such lenses.
Accordingly, there is provided a novel system and design for permitting alignment of a contact lens to a given rotational orientation in the eye by the natural blinking action of the eyelid using at least one alignment groove, preferably on the exterior surface of the lens.